eCryptfs: Support metadata in xattr

This patch set introduces the ability to store cryptographic metadata
into an lower file extended attribute rather than the lower file
header region.

This patch set implements two new mount options:

ecryptfs_xattr_metadata
 - When set, newly created files will have their cryptographic
   metadata stored in the extended attribute region of the file rather
   than the header.

ecryptfs_encrypted_view
 - When set, this option causes eCryptfs to present applications a
   view of encrypted files as if the cryptographic metadata were
   stored in the file header, whether the metadata is actually stored
   in the header or in the extended attributes.